Because it's not going to have an electron configuration of #1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 4p^6# in the ground state... iron has #3d# orbitals available... the energy ordering of the #4s# and #3d# may be lacking consistency among some textbooks, but certainly not the #4p# and #3d#.
#overbrace(2)^(1s^2), overbrace(8)^(2s^2 2p^6), overbrace(14)^(3s^2 3p^6 3d^6), overbrace(2)^(4s^2 4p^0)#
#K, " "L, " "M, " "N#
Iron cannot skip the #3d# orbitals and use #4p# orbitals. We know already that the #4p# orbitals are higher in energy than the #3d# orbitals, by virtue of the #4p# having a higher #l# than the #4s#, and the #4s# being at least CLOSE in energy to the #3d#.
Hence, the #3d# orbitals are occupied (and NOT the #4p#!), as expected by the Aufbau principle (iron is not some weird exception here!).
The #3d# orbitals are in the so-called "#M# shell", so in "#KLM#" notation (which is not particularly clear, but is nevertheless used for X-ray experiments), one would write #color(blue)(2,8,14,2)#.
#overbrace(2)^(1s^2), overbrace(8)^(2s^2 2p^6), overbrace(14)^(3s^2 3p^6 3d^6), overbrace(2)^(4s^2 4p^0)#
#K, " "L, " "M, " "N#
